LHC’s First Act: Higgs Found, Other Weird Physics Still at Large

The list of important things that the LHC found in its initial run can be summed up in one word: Higgs. Though scientists remain a bit coy about calling the particle they found in July 2012 the Higgs boson (more data is needed to conclusively prove this) it is something that looks like the Higgs, acts like the Higgs, and was found where the Higgs was expected. Given that this long-sought boson is all but officially discovered, it can take its place as the final piece in the puzzle of physicists’ Standard Model — the established theory explaining the interactions of all known particles and forces.

While finding the Higgs has been a triumph concluding one generation of physicists' wildest dreams, scientists can rightfully said to be a little disappointed. That’s because the LHC was not built just to hunt down one particular boson. It was meant to uncover a host of new subatomic particles and exotic phenomena. In this gallery, we will take a look at some of these hoped-for events and what they might have meant for science had they been found.

If the Higgs is all that the LHC discovers “it could be a huge disaster,” said theoretical physicist Lawrence Krauss of Arizona State University.

Getting a look at the Higgs was supposed to give scientists a glimpse of physics beyond the Standard Model, which has problems and holes that prevent it from being the final theory of everything. But the Higgs has been stubbornly normal, more or less what was expected from Standard Model predictions with barely a hint of anything revolutionary.

“All the important question remain unknown and we’re waiting for some new information about the Standard Model and which direction to go,” said Krauss.

In particular, physicists had by now hoped to see some evidence of a theory known as supersymmetry, which predicts the existence of an almost identical but heavier partner to every known subatomic particle. Supersymmetry elegantly solves many of the Standard Model’s problems but, as yet, there has been no real verification of its mechanisms and physicists are beginning to lose faith in the idea that actually exists.

But the possibility of seeing either supersymmetry or other new physics is not yet completely out of the question. When the LHC comes back online in December 2014, it will be colliding protons at twice the energy that it has so far achieved. This will allow scientists to probe higher and higher energy ranges and potentially see the particles and other forces that didn’t turn up during this initial run.

“Experimenters are clever and careful,” said physicist Martin Perl, who shared the Nobel Prize in 1995 for discovering the tau particle. “There’s a lot more to discover but we’ll have to wait and see.”

Image: A technician welding inside the beam tube at the Large Hadron Collider. CERN

Two Higgses

Back in July, the only thing that might have been better than finding one Higgs was to have seen two Higgs bosons. The two Higgses — actually the Higgs boson and its supersymmetric partner — would have been an even more monumental discovery, since it would have found the long-sought boson and confirmed the existence of supersymmetry.

If supersymmetry is correct, scientists will eventually see another Higgs-type particle when they probe higher energies — it will just be a much heavier partner to the Higgs seen in July. In the meantime, physicists will pick over every single detail of the Higgs to see if it differs from what the Standard Model predicts and could lead to interesting new physics.

Supersymmetric Superpartners

It was hoped that by now physicists would have seen the lightest sorts of particles that could confirm supersymmetry. The Standard Model already has plenty of subatomic particles within it, like the well-known electrons and neutrinos. Supersymmetry would give each of these particles a partner with almost identical properties but a heavier mass.

Since there are many different supersymmetric theories, no one knows exactly what the mass of any of these partners might be. Had the LHC found one, it would have put constraints on theoretical models and perhaps given scientists hints of where to look next for more evidence of supersymmetry. Given that none have been seen, the supersymmetric partners must be much heavier than initially thought, which has forced physicist to include ugly workarounds that have dampened some of the theory’s appeal.

There is still a chance that some of these particles will show up during the LHC’s next run. If they don’t and supersymmetry is finished, it will drive physics into a crisis over how to explain the discrepancies of the Standard Model.

Image: Simulated particle events that would indicate the existence of supersymmetry. CMS experiment

Dark Matter

Part of supersymmetry’s appeal is that it contains so many new particles. Perhaps one of them might have the right properties to explain dark matter — an as-yet-unknown substance that makes up more than 80 percent of all matter in the universe. Astronomers have ample evidence of its existence, and have shown that it interacts gravitationally with ordinary matter, but have no real idea what exactly it might be.

The leading candidate for dark matter is a supersymmetric partner to the neutrino called the neutralino. This particle would be much heavier than the neutrino but would fly through ordinary matter as easily as a bullet through thick fog. There are currently many searches underway to directly detect such particles, watching a large mass of atoms until one of them gets hit, but it’s possible that the LHC could have produced one by now.

No neutralinos have been seen at the LHC but scientists are not too worried. Dark matter exists and it will only be a matter of time before we are able to see it in a laboratory.

New Forces and Generations

According to the Standard Model, subatomic particles are arranged in three groupings known as generations, with each generation heavier than the last. Electrons, for instance, belong to the lightest generation of particles but interact similarly to the heavier muons and tau particles, which belong to the second and third generation, respectively.

But why are there only three generations? Scientists have never seen any evidence for higher generation particles but that doesn’t necessarily mean they don’t exist. Though unlikely, it was possible that the LHC could have uncovered evidence for a fourth generation, perhaps by finding a new type of never-before-seen neutrino. The best evidence against this was the discovery of the Higgs boson. If there were higher-generation particles, they would have interacted with the Higgs and modified its properties from what scientists saw in July.

Similarly, there are four fundamental forces in the universe: the electromagnetic, strong, weak, and force of gravity. No other forces have ever been detected but they could still be out there. Some slight evidence suggests that dark matter may interact with itself via fundamental forces that ordinary matter lacks. Such “dark forces” could explain certain phenomena seen in small galaxies. If the LHC had seen either supersymmetric particles or dark matter, it might have also glimpsed some new and very different force of nature.

Image: The three generations of matter get heavier from left to right. The four fundamental force-carrying particles are at the farthest right. MissMJ/Wikimedia

More Exotic Phenomena

There was a very slim chance that the LHC could have completely surprised everyone and seen evidence for some extremely weird and exotic phenomena. This could have included new dimensions posited by string theory or something helping to explain the nature of dark energy, the mysterious force causing an accelerated expansion of the universe.

“People would have been amazed had they been seen,” said theoretical physicist Lawrence Krauss. “But those things were always a real long shot.”

Scientists will always want to probe farther and understand how the universe is put together. If during its upgraded run, the LHC sees evidence for things like supersymmetry, it will help make the case for more experiments that will lead to more discoveries.

“We’re just limited by our technology at any given time,” said physicist Martin Perl. ”I lean towards the idea that we’re just children and 100 years from now they’ll laugh at us and what we didn’t know.”